The regulation of the expression of nucleolar genes in hybrids of Xenopus species is being examined from two aspects. In the first, attempts are being made to characterize chromosomal proteins at the nucleolar sites and to compare them to their counterparts in the parental species. In the second, hybrid cells are being cultured in vitro in an attempt to control nucleolar expression by added cell fractions from embryos of the parental species. Synchronously-fertilized Xenopus eggs are also being examined for the effect of aging and temperature shocks applied to the cleaving egg on the development of reproductive potential. The assay system is karyotypic and histological.